1. Field of the Invention
The present invention relates generally to a mold for the molding of disc substrata for video games, compact discs, or memory discs for digital data (all hereinafter described as discs) and particularly concerns a mold for molding discs with a novel outer peripheral ring having such a configuration as to improve the streams of blowing air for releasing the molded discs from the stamper set on the surface of a mold cavity.
2. Description of the Related Art
In a prior art mold for molding discs, molds having such an outer peripheral ring such as shown in U.S. Pat. No. 5,297,951 have been disclosed.
An outer peripheral ring like that disclosed in U.S. Pat. No. 5,297,951 is shown in FIG. 5. The inclined large inner peripheral surface 80a of the outer peripheral ring 80 closes on the outer peripheral surface 62a of a mirror plate 62 fixed to a movable moldhalf so as to define the position of the outer peripheral ring 80 in the movable moldhalf. A portion of the inclined small inner peripheral surface 85 defines the outer peripheral surface of a mold cavity 61, of which a remaining portion overlaps a portion of the outer peripheral surface of the stationary mirror plate 63, maintaining a slight gap 50 between both when mold is closed as shown in FIG. 5.
The outer peripheral ring 80 has an air passage 87 through the interior to release molded discs from the stamper 70 by blowing air through a slight gap S of 0.01-0.04 mm between the underlying surface 81 of the inclined small inner peripheral surface 85 and a surface of a stamper 70, which is attracted to the surface of the mirror plate 62 by suction of air through air passage 66.
FIG. 6 shows how blowing air releases the molded discs. Melted resin has been injected into the mold cavity 61 to be solidified. Inevitably, molded discs D shrink in the mold cavity 61 due to cooling. For example, compact discs, the diameter and thickness of which are 120 mm and 1.2 mm respectively, have shrinkage of the diameter different from shrinkage in the thickness. As shrinkage of polycarbonate resin is about 0.6% in the molding of compact discs, the amounts of the shrinkage of the molded discs in diameter and in thickness are 0.72 mm and 0.0072 mm respectively.
In FIG. 6, Y designate the amount of shrinkage of a radius and X designates shrinkage of thickness. Y is considerably larger than X. Consequently, when blowing air through passage 87, air is apt to be released towards and outer space 90 through the gap 50. Accordingly, the amount of blowing air effective for releasing the molded disc D from the stamper 70 is reduced, bringing about such undesirable effects on the quality of the molded discs as stain marks or cloud.